Research Article pubs.acs.org/journal/ascecg
Cite This: ACS Sustainable Chem. Eng. 2019, 7, 11150−11156
Depolymerization of Laccase-Oxidized Lignin in Aqueous Alkaline Solution at 37 °C Haifeng Liu,† Leilei Zhu,†,● Anne-Maria Wallraf,† Christoph Räuber,‡ Philipp M. Grande,§ Nico Anders,∥ Christoph Gertler,† Bernd Werner,⊥ Jürgen Klankermayer,§ Walter Leitner,§,# and Ulrich Schwaneberg*,†,■ †
Institute of Biotechnology, RWTH Aachen University, Worringerweg 3, 52074 Aachen, Germany Institute of Organic Chemistry, RWTH Aachen University, Landoltweg 1, 52056 Aachen, Germany § Institut für Technische und Makromolekulare Chemie (ITMC), RWTH Aachen University, Worringerweg 1, 52074 Aachen, Germany ∥ Aachener Verfahrenstechnik−Enzyme Process Technology, RWTH Aachen University, Worringerweg 1, 52074 Aachen, Germany ⊥ Institute of Chemistry, University of Graz, Heinrichstraße 28/II, 8010 Graz, Austria # Max Planck Institute for Chemical Energy Conversion, Stiftstraße 34-36, 45470 Mülheim a.d. Ruhr, Germany ■ DWI an der RWTH Aachen e.V., Forckenbeckstraße 50, 52056 Aachen, Germany ● Tianjin Institute of Industrial Biotechnology, Chinese Academy of Sciences, 300308 Tianjin, China
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‡
S Supporting Information *
ABSTRACT: Lignin depolymerization is a coveted process for the generation of high-value compounds from a low-cost feedstock. This report describes a chemoenzymatic approach for the depolymerization of lignin involving a laccase-catalyzed oxidation under ambient air at room temperature followed by a base (NaOH)-induced depolymerization in aqueous solution at 37 °C. Two-dimensional nuclear magnetic resonance heteronuclear single quantum coherence spectroscopy, gel permeation chromatography, and liquid chromatography-electrospray ionization-quadrupole-time of flight-mass spectrometry (LC-ESIQ-TOF-MS) analysis indicated the degradation of lignin and the formation of water-soluble fractions containing guaiacol, syringol, vanillic acid, m-anisic acid, and veratric acid. Furthermore, guaiacol and veratric acid are the main final products in the chemoenzymatic decomposition of a β-O-4 model compound. KEYWORDS: Lignin depolymerization, Laccase, Oxidation, Alkaline treatment, Chemoenzymatic approach
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INTRODUCTION
sustainable manner have focused on catalytic oxidative transformations7−17 and also catalytic cracking,18 hydrolysis,19 acid catalysis,20−22 base catalysis,23,24 reduction,25−27 and transfer-hydrogenation28−30 processes. Recently developed organocatalyzed oxidations of the Cα alcohols to ketones in native lignin have enabled the subsequent chemical treatment of this oxidized lignin for selective depolymerization, based on C−O bond cleavage induced by photocatalysts,31 zinc,32 or formic acid.33 In these organocatalytic oxidation procedures, chemical oxidants (Bobbitt’s salt,31 DDQ,32 and HNO334) are necessary for the chemoselective oxidation of secondary benzylic alcohols in β-O-4 linkages.
The transformation of industry toward a sustainable biobased economy (bioeconomy) is a major challenge of the 21st century. One approach to achieve this goal within the next decades is the accession of previously neglected biological resources, for example, lignin. Lignin represents 30% of the organic carbon in plant biomass.1 This biopolymer consists of phenyl propane units and represents a promising source for both renewable fuels and aromatic chemicals. Yet, most of the lignin currently produced from the pulp and paper industry currently is burned as low-value fuel because of its complex chemical structure.2 The plant species,3 biomass pretreatment method,4 and pretreatment severity5 affect the structure and chemical composition of the lignin,6 where the predominant interconnecting bond in wood-derived lignin is the β-O-4 alkyl-aryl ether linkage. Efforts to make use of lignin in a more © 2019 American Chemical Society
Received: January 11, 2019 Revised: May 28, 2019 Published: June 3, 2019 11150
DOI: 10.1021/acssuschemeng.9b00204 ACS Sustainable Chem. Eng. 2019, 7, 11150−11156
Research Article
ACS Sustainable Chemistry & Engineering Scheme 1. Reaction Scheme of the Two-Step Chemoenzymatic Lignin Depolymerization
Scheme 2. Laccase-Initiated Decomposition of Adlerol (1) with the Formation of Cleavage Products Veratric Acid (4) and Guaiacol (5)
“recycling” of the laccase, violuric acid mediator, and [EMIM] [EtSO4]. In step II, NaOH mediated the dehydration of βhydroxyketone 2 to aryl vinyl ether intermediate 3, which was subsequently cleaved to yield veratric acid (4) and guaiacol (5) as the main final products at 37 °C in the presence of ambient air (step III). As no cleavage products were observed when adlerol 1 was directly subjected to the alkaline conditions of step II, the chemoselective enzymatic oxidation of adlerol 1 by the laccase lcc2M3 was considered essential for the succeeding decomposition. The key intermediate 3 can be generated only by dehydration when the Cα alcohol is oxidized to the ketone because of the reaction mechanism of an aldol condensation. Recent attempts to degrade lignin under basic conditions15,23,24,43−45 have shown that elevated temperature and/ or pressure were necessary. As mentioned above, βhydroxyketones have been frequently described as oxidation products in oxidative transformations. The formation of arylvinyl-type ethers in the cleavage mixture of β-O-4 model compounds was mentioned in 201146 and characterized in 2013.34,47 Stahl and co-workers reported in 2014 that the formation of the aryl vinyl ether intermediate 3 is the key step for lignin depolymerization under the conditions selected for their respective study.33 This study shows that the role of the base in the depolymerization of oxidized lignin is both to induce a dehydration reaction of β-hydroxyketones (step II) based on aldol condensation and to facilitate the successive decomposition of the α,β-unsaturated ketone (step III). Investigation on the mechanism of the base-mediated cleavage of enone 3 is in progress. In addition to the employed model compound adlerol 1, this two-step chemoenzymatic strategy was successfully applied to the decomposition of beech wood lignin separated from a scalable, recyclable, and compatible (for different biomass sources) fractionation process,48 “OrganoCat process”,49 which is based on selective acid-catalyzed depolymerization in a biphasic water−organic solvent system to separate lignin in a single processing step.50 The same reaction conditions were chosen as for adlerol 1 (Scheme 2) while omitting organic solvents in step II and step III as the oxidized lignin was readily
Ambient air35 as an inexpensive and environmentally benign oxidizing agent can serve as an oxidant in a laccase-mediator system (LMS) for the conversion of Cα alcohols to ketones in β-O-4 model compounds.36,37 Laccases are involved in lignin degradation in nature.38 Yet, it remains unclear whether and how laccases can directly catalyze the bond decomposition of lignin. A multitude of investigations have demonstrated that laccase treatment in fact resulted in the polymerization of various lignin samples to higher masses.39 Another challenge for the application of a LMS in the oxidative transformation of lignin maintains its resistance toward cosolvents (e.g., ionic liquids) that are used to dissolve lignin. In an earlier work,40 we reported on the laccase variant lcc2M3 with an improved resistance toward [EMIM] [EtSO4], an ionic liquid,41 effective for the solubilization of lignin. Although recent reports on the organocatalyzed oxidation of Cα alcohols in β-O-4 linkages enabled the chemical treatment of oxidized lignin to aromatics, the pivotal challenge for selective lignin depolymerization is to develop a facile procedure for the covalent bond cleavage of β-O-4 linkages under mild conditions. Here, we describe a two-step lignin degradation strategy (Scheme 1) which employs an engineered laccase (variant lcc2M3) to catalyze the chemoselective oxidation of lignin followed by a base-induced depolymerization in aqueous solution.
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RESULTS AND DISCUSSION A chemoenzymatic procedure was developed to facilitate the decomposition (Scheme 2) of adlerol 1, a commonly used βO-4 model compound.42 In step I, the secondary benzylic alcohol in adlerol was converted into the corresponding ketone 2 by the laccase variant lcc2M3. Thereby, ambient air was employed as oxidant and violuric acid as mediator. The enzymatic oxidations of adlerol and simple benzyl alcohols (Gtype and S-type nonphenolic model compound, Table S1, Supporting Information) were performed at ambient temperature and in the presence of 10% (v/v) [EMIM] [EtSO4]. The majority of the resulting adlerone 2 precipitated from the reaction solution because of the decreased polarity of the product after lcc2M3 oxidation. The precipitation of adlerone 2 substantially simplified the product purification and 11151
DOI: 10.1021/acssuschemeng.9b00204 ACS Sustainable Chem. Eng. 2019, 7, 11150−11156
Research Article
ACS Sustainable Chemistry & Engineering
Figure 1. 2D-HSQC-NMR spectra of OrganoCat lignin in d6-DMSO: (A) OrganoCat lignin before the reaction; (B) precipitated laccase-oxidized OrganoCat lignin (76 wt % of original OrganoCat lignin).
protons in β-O-4 linkage were consumed (β-O-4 content changed from 41.9 to 17 because of oxidation), which indicated an excellent preference of the laccase lcc2M3 for the oxidation of benzylic alcohols within the β-O-4 linkages. Gel permeation chromatography (GPC) analysis of the oxidized lignin (blue line in Figure 2) showed that the OrganoCat lignin was partially converted to structures with higher molecular mass after the LMS-catalyzed oxidation.53,54 This is likely caused by laccase-induced radical polymerization of the phenolic fragments38,55 within lignin. In addition to the change in polarity, the increased mass could also be responsible for the precipitation of the oxidized lignin from the enzymatic oxidation solution that contains 10% (v/v) [EMIM] [EtSO4]. After the alkaline treatment, the mass maximum of the residual water-insoluble fraction of lignin (green line in Figure 2) reduced from ∼3500 Da (native lignin) to ∼2000 Da. Therefore, it was hypothesized that the majority of the residual water-insoluble fractions of lignin were oligomers. The enzymatically oxidized lignin was depolymerized to lowmolecular-mass aromatics (water-soluble fraction) after alkaline treatment at 37 °C, as can be seen in the results of highresolution mass spectrometry coupled with liquid chromatography (LC-ESI-Q-TOF-MS) that was used to identify watersoluble low-molecular-mass aromatics (Table 1).
soluble in NaOH aqueous solution. As observed in the modelbased studies, the oxidized lignin precipitated from a 10% (v/ v) [EMIM] [EtSO4] solution in step I and no further precipitation of oxidized lignin from the rest of the reaction mixture occurred after dilution with water. On the basis of the reaction mechanism that no C−O or C−C bond cleavage occurred during LMS-catalyzed oxidation, we believe that there was no gaseous side products formed during enzymatic oxidation theoretically. This precipitation facilitates the separation of the oxidized lignin and enables the reuse of both the ionic liquid (IL) and LMS. For lignin from a different source and pretreatment process, the final IL concentration can be adjusted to perform fully enzymatic oxidation in a ILassisted homogeneous reaction system. According to the two-dimensional nuclear magnetic resonance heteronuclear single quantum coherence (2DHSQC NMR) spectra (OrganoCat lignin, Figure 1, β-O-4 model compound, Figure S1, and DDQ51 oxidized lignin, Figure S2), in LMS-treated lignin sample, the enzymatic oxidation of Cα alcohol in β-O-4 linkage was not complete, but β−β linkage (structure C in Figure 1) remained unaffected. On the basis of signal intensity,17,52 further characterization of linkages (Figure S3a,b and Table S2) showed that, after enzymatic oxidation, most β−β linkages (>95%, 4.8 linkages per 100 C9 units) were maintained; meanwhile, 60% α11152
DOI: 10.1021/acssuschemeng.9b00204 ACS Sustainable Chem. Eng. 2019, 7, 11150−11156
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ACS Sustainable Chemistry & Engineering
Veratric acid (4, 0.9%) and guaiacol (5, 0.1%) were detected as low-molecular-mass aromatics of the depolymerized OrganoCat lignin (Table 1). These findings are in accordance with the final products obtained during the decomposition of β-O-4 model compound adlerol 1. The production of syringol (8), m-anisic acid (7, 0.5%), and vanillic acid (9, 3.9%) in the water-soluble fraction can be explained by the same baseinduced decomposition procedure. After 6 h of alkaline treatment, most of the detected low-molecular-mass aromatics are G-type compounds, likely because S lignin units consisted of water-insoluble fraction (large-molecular-weight fragments56).
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CONCLUSIONS A two-step process involving an enzyme-initiated oxidation and a base-induced cleavage strategy was developed for the selective depolymerization of lignin. In this mild degradation process, a laccase variant catalyzes the essentially selective αoxidation of the β-O-4 linkage to β-hydroxyketones at room temperature, followed by the formation of the key intermediate, aryl vinyl ether 3, through base-induced elimination in aqueous solution. The final depolymerization products were formed through decomposition of the aryl vinyl
Figure 2. Gel permeation chromatography (GPC) of OrganoCat lignin; native OrganoCat lignin (black); oxidized OrganoCat lignin (76 wt % native OrganoCat lignin, blue); water-insoluble fraction (51 wt % enzymatic oxidized OrganoCat lignin, precipitated from NaCl saturated reaction mixture, green).
Table 1. LC-ESI-Q-TOF-MS Analysis of the Low-Molecular-Mass Aromatics Produced in the Depolymerization of OrganoCat Lignin
a
Yield of low-molecular-mass aromatics to oxidized lignin was determined by HLPC measurement. 11153
DOI: 10.1021/acssuschemeng.9b00204 ACS Sustainable Chem. Eng. 2019, 7, 11150−11156
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ACS Sustainable Chemistry & Engineering ether intermediate. To the best of our knowledge, this is the first time that an enzymatic oxidation (at room temperature) is cascaded with an alkaline treatment (at 37 °C) for the efficient lignin depolymerization. The investigation on this mild chemoenzymatic procedure improves our insight into the role of the laccase enzyme in natural lignin degradation. Furthermore, the results of this study may enable the generation of valuable chemical compounds from a waste feedstock as part of a biobased economy.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] (U.S.). ORCID
Ulrich Schwaneberg: 0000-0003-4026-701X
EXPERIMENTAL SECTION
Notes
Chemoenzymatic Decomposition of Adlerol 1. A 15 mL Falcon tube was charged with the lignin model compound adlerol 1 (33.4 mg, 100 μmol, 1 equiv), laccase lcc2-M3 (1 unit), and violuric acid monohydrate (18 mg, 103 μmol, 1.03 equiv) in 3.15 mL of aqueous sodium acetate solution (0.1 M, pH 4) and 0.35 mL of [EMIM] [EtSO4] (99%, Iolic tec GmbH, Germany). The reaction mixture was incubated at room temperature under shaking (250 rpm) for 3 days. A white solid precipitated over the course of the reaction and was collected by filtration, washed several times with water, and dried under high-vacuum conditions. Adlerone 2 (16.6 mg, 50 μmol, 1 equiv) was weighed in a 15 mL Falcon tube, and 1 mL of NaOH in D2O (1 M) and 9 mL of CD3OD were added. The reaction mixture was stirred at 37 °C for 1 h. After the reaction time, the mixture was transferred to an NMR tube and directly analyzed by NMR spectroscopy utilizing duroquinone as the internal standard. Aryl vinyl ether compound 3 (9.4 mg, 30 μmol, 1 equiv) was reacted in Eppendorf tubes with a NaOH ethanol solution (1 M, 1 mL/tube ×10 tube, tubes were kept open) on an Eppendorf Thermomixer 5436 at 37 °C overnight. The residual reaction mixture was dissolved in water, adjusted to pH 3, and then extracted with EtOAc (3 × 10 mL). The combined organic phase was washed with brine and dried over MgSO4 and the solvent was removed under reduced pressure. To the reaction solution 1.000 mL of a standard solution (3,4dimethoxybenzylalcohol in methanol, c = 0.2 mol/L) was added with an Eppendorf pipet. For HPLC measurements, 2−3 mg of the reaction product was weighed into a vial. Then 0.5 mL of ethyl acetate and 0.5 mL of acetonitrile were added to the vial, and after the complete dissolution, the solution was filtered into a HPLC vial. Oxidation and Depolymerization of OrganoCat Lignin Samples. The dry lignin sample (300 mg, separated from beech wood), violuric acid monohydrate (108 mg), and 6 units of lcc2M3 were added to 6 mL of [EMIM] [EtSO4] and 54 mL of aqueous sodium acetate solution (0.1 M, pH 4). The reaction mixture was incubated at room temperature for 3 days. Afterward, the solid in the reaction mixture was collected by centrifugation (16 100g, 4 °C) and washed twice with ddH2O. The collected solid (76 wt % of native OrganoCat-lignin) was directly dissolved in 200 mL of an aqueous sodium hydroxide solution (5 M). After incubation at 37 °C for 6 h, the reaction mixture was acidified to pH 3 and then saturated with NaCl. The water-insoluble residue (51 wt % of enzymatic oxidized OrganoCat-lignin) was separated by centrifugation (16 100g, 4 °C), then dissolved in ethyl acetate (4 × 25 mL), and concentrated under vacuum for GPC analysis. Meanwhile, the aqueous solution was extracted with ethyl acetate (4 × 25 mL); the organic phases were combined, dried over anhydrous MgSO4, filtered, and concentrated under vacuum. The residual low-molecular-mass aromatics (42 wt % of enzymatic oxidized OrganoCat-lignin) were analyzed by LC-ESIQ-TOF-MS and HPLC.
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compound, DDQ oxidized OrganoCat lignin based on HSQC spectra; GPC measurements for the OrganoCat lignin samples; LC-MS and HPLC measurements for the OrganoCat lignin samples (PDF)
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was conducted as part of the Cluster of Excellence “Tailor-Made Fuels from Biomass”, funded by the Excellence Initiative of the German Research Foundation to promote science and research at German universities. Furthermore, financial support from DFG training group 1166 “BioNoCo” (“Biocatalysis in Non-conventional Media”) is acknowledged as well. C. Bolm and his co-workers, J. Mottweiler, and T. Rinesch kindly provided the lignin model compound and read and corrected this paper. H.L. thanks A. Spieß, N. Harwardt, and S. Roth (RWTH Aachen University) for initial scientific discussion in Aachen. The ElkGroup (W. Kroutil/K. Faber group, University of Graz) is acknowledged for great support during paper preparation and revision in Graz.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.9b00204. Spectroscopic data of the isolated products; characterization of the OrganoCat lignin samples, β-O-4 model 11154
DOI: 10.1021/acssuschemeng.9b00204 ACS Sustainable Chem. Eng. 2019, 7, 11150−11156
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DOI: 10.1021/acssuschemeng.9b00204 ACS Sustainable Chem. Eng. 2019, 7, 11150−11156
